给定一个返回Future
的方法......
def remove(id: String): Future[Option[User]] = Future {
// removes and returns the user identified by `id`
}
...如何一次又一次地调用它,直到它返回包含Future
的{{1}}值?
修改
也许值得一提的是,我不需要收集结果。我只需要找到要删除的用户就可以调用该方法。我们的想法是None
在loop
返回remove
时停止。
答案 0 :(得分:4)
之前有人评论说没有意义。
让我感到意外的是,懒散地消费期货并不是一件容易的事。 Future.find
与firstCompletedOf
类似,但并不代表find first in traversable order
。
scala> import concurrent._, ExecutionContext.Implicits._
import concurrent._
import ExecutionContext.Implicits._
scala> import java.util.concurrent.atomic._
import java.util.concurrent.atomic._
scala> val count = new AtomicInteger(10)
count: java.util.concurrent.atomic.AtomicInteger = 10
scala> def f(s: String) = Future { if (count.decrementAndGet <= 0) None else Some(s) }
f: (s: String)scala.concurrent.Future[Option[String]]
scala> def g(ss: List[String]): Future[List[String]] = f("hello") flatMap { case None => Future.successful(ss) case Some(s) => g(s :: ss) }
g: (ss: List[String])scala.concurrent.Future[List[String]]
scala> g(Nil)
res0: scala.concurrent.Future[List[String]] = scala.concurrent.impl.Promise$DefaultPromise@65a15628
scala> .value
res1: Option[scala.util.Try[List[String]]] = Some(Success(List(hello, hello, hello, hello, hello, hello, hello, hello, hello)))
说明不阻止的效用:
scala> :pa
// Entering paste mode (ctrl-D to finish)
import scala.util._
import concurrent._, ExecutionContext.Implicits._
import java.util.concurrent.atomic._
class Work {
val count = new AtomicInteger(10)
def f(s: String) = Future {
if (count.decrementAndGet <= 0) None else Some(s)
} andThen {
case Success(Some(x)) => Console println s"Calculated $x"
case Success(None) => Console println "Done."
case _ => Console println "Failed."
}
}
// Exiting paste mode, now interpreting.
import scala.util._
import concurrent._
import ExecutionContext.Implicits._
import java.util.concurrent.atomic._
defined class Work
显示Stream
版本,在消费线程逐步执行阻止之前不会计算前缀等待:
scala> val work = new Work
work: Work = Work@1b45c0e
scala> Stream continually work.f("hello") takeWhile { x => Await.result(x, duration.Duration.Inf).nonEmpty }
Calculated hello
res0: scala.collection.immutable.Stream[scala.concurrent.Future[Option[String]]] = Stream(scala.concurrent.impl.Promise$DefaultPromise@66629f63, ?)
scala> .toList
Calculated hello
Calculated hello
Calculated hello
Calculated hello
Calculated hello
Calculated hello
Calculated hello
Calculated hello
Done.
res1: List[scala.concurrent.Future[Option[String]]] = List(scala.concurrent.impl.Promise$DefaultPromise@66629f63, scala.concurrent.impl.Promise$DefaultPromise@610db97e, scala.concurrent.impl.Promise$DefaultPromise@6f0628de, scala.concurrent.impl.Promise$DefaultPromise@3fabf088, scala.concurrent.impl.Promise$DefaultPromise@1e392345, scala.concurrent.impl.Promise$DefaultPromise@12f3afb5, scala.concurrent.impl.Promise$DefaultPromise@4ced35ed, scala.concurrent.impl.Promise$DefaultPromise@2c22a348, scala.concurrent.impl.Promise$DefaultPromise@7bd69e82)
scala> .foreach (Console println _.value.get)
Success(Some(hello))
Success(Some(hello))
[snip]
其他行为,可能更合适,你得到一个包含计算前缀结果的Future:
scala> :pa
// Entering paste mode (ctrl-D to finish)
val work = new Work
def g(ss: List[String]): Future[List[String]] = work.f("hello") flatMap {
case None => Future.successful(ss)
case Some(s) => g(s :: ss)
}
// Exiting paste mode, now interpreting.
work: Work = Work@796d3c9f
g: (ss: List[String])scala.concurrent.Future[List[String]]
scala> g(Nil)
Calculated hello
Calculated hello
res3: scala.concurrent.Future[List[String]] = scala.concurrent.impl.Promise$DefaultPromise@99a78d7
Calculated hello
Calculated hello
Calculated hello
scala> Calculated hello
Calculated hello
Calculated hello
Calculated hello
Done.
使用未来:
scala> .value
res5: Option[scala.util.Try[List[String]]] = Some(Success(List(hello, hello, hello, hello, hello, hello, hello, hello, hello)))
答案 1 :(得分:0)
Stream#continually
无休止地做同样的事情,Stream#takeWhile
在某个时刻停止它。
http://www.scala-lang.org/api/2.11.0/index.html#scala.collection.immutable.Stream
Stream.continually(/*remove*/).takeWhile(/*not Future[None]*/)
答案 2 :(得分:0)
这是:
import concurrent._, ExecutionContext.Implicits._
import java.util.concurrent.atomic._
val count = new AtomicInteger(10)
def f(s: String) = Future {
if (count.decrementAndGet <= 0) None else Some(s)
}
Iterator continually {
f("hello")
} takeWhile {
Await.result(_, duration.Duration.Inf).nonEmpty
} foreach { _.map { _.map {
println
}}
我希望它有所帮助。